If your question is specifically on vuln scanners, then Nessus is just fine. Doesn't have the bells and whistles as some of the big boys like ISS, LANGuard, etc. But considering that many companies have nothing, making yourself knowledgeable in Nessus will do you well. Plus, if being hired is your concern, I don't think anyone will look down on you for only knowing Nessus. Not knowing any vuln scanner can hurt you.

As for Core Impact, it is an automated pen testing tool and much less of a vuln scanner. According to their web site:

Those free utilities are great. It really depends on what you are trying to do as far as a hack is concerned and your client. You have to remember that 99% of the hacking going out there is done with those free tools. Hackers usually can’t afford and therefore don’t use tools like core, at least as far as I have seen. If you are tying to secure a network from hackers then you should use the same approach most of them do.

If you assault a network skillfully with the traditional tools and also know how to compile exploits and you cant get in, then you can feel assured 90% of what is out there cant get in either. If I where doing security work for someone like Amazon.com that has millions of dollars at stake, well Core would be a must. Some of the people in the DOD use Canvass and seem to like it a lot. In other words, if you can’t afford expensive tools, please don’t think for a second you can’t do some very effective hacking!

Thanks for the replies. I guess I just thought if I ever apply for a pentester position, they would mainly be looking for those commercial tools like CORE Impact and Canvass if that's what I'd be using with them. I can see how Nessus and those other free tools would be beneficial to have on the resume as well. Keeps you at the level of the malacious hacker.